As a new user, the first thing I did was to clutter up my sketch collection with a half dozen “new” (cloned) sketches of various types. Now I’m left wondering how to delete them. It must be something obvious like a “delete me” on the edit menu or something; but I don’t see anything like that.

you can delete sketches with a long-press. It should bring up a menu that has a delete option. If you’re using iCloud you can select if you want to remove the local version only or also delete the sketch from iCloud.